SUMMARY Functionally distinct populations of murine intestinal stem cells (ISCs), including rapidly cycling crypt base columnar (CBC) ISCs and dormant ISCs (d-ISCs), maintain the highly self-renewing intestinal epithelium. While CBC ISCs play an important role in daily homeostasis, d-ISCs are highly resistant to injury and play an important role during intestinal regeneration. Precisely what cellular and molecular mechanisms regulate the regenerative response of d-ISCs remains largely unknown. Regulated by complex and still poorly understood mechanisms, fasting triggers intestinal injury from which the intestine recovers during re-feeding mediated by d-ISCs. Fasting/re-feeding is therefore a useful tool to study the mechanisms that regulate the d-ISC response to injury. We recently showed that fasting leads to a dramatic induction in d-ISC number and regenerative response, associated with a transient inactivation of PTEN, a negative regulator of the PI3KAKTmTORC1 pathway. In contrast, permanent deletion of PTEN led to a complete loss of d-ISCs, due in part to unrestrained PI3K signaling. This phenotype could be rescued by inhibition of PI3K signaling and increases in canonical Wnt/?catenin (cWnt) signaling. Exactly what downstream effectors of PI3K and cWnt signaling regulate the d-ISC regenerative response remains unclear. Given these findings, this proposal will: 1) establish the role of PI3KAKTmTORC1 signaling in the d-ISC injury response, 2) define the role of cWnt signaling in d-ISC survival/maintenance following PTEN loss, and 3) identify downstream effectors of PI3K and cWnt signaling pathways that regulate the d-ISC regenerative response to injury.